1. Field of the Invention
The present invention relates to power semiconductor devices and particularly to a device having a current limiter of an output transistor.
2. Description of the Related Art
A power semiconductor device includes an input circuit, an output metal oxide semiconductor (MOS) transistor, an output MOS transistor control circuit, and so on. The power output MOS transistor heats up if a high current flows and energy increases. Thus, overcurrent causes thermal destruction of the output MOS transistor.
To avoid the thermal breakdown, some output MOS transistor control circuits have a current limiter. The current limiter monitors the current of the output MOS transistor and controls the gate voltage to limit an output current. The current limiter thereby prevents the self-destruction of the output MOS transistor due to high energy.
Since a higher drain-source voltage causes a higher energy in the output MOS transistor, it is necessary to reduce a current limit value as the drain-source voltage increases. Thus, development of a current limiter that allows the current control proportional to the drain-source voltage has been required.
FIGS. 6 and 7 illustrate examples of current limiters. In FIG. 7, the same reference symbols as in FIG. 6 designate the same elements, and redundant explanation is omitted. The current limiter includes an Nch source follower in which an output MOS transistor 64 and a load 68 are connected in series between a first power supply 61 and a second power supply 62. An output terminal 69 is connected to a connection node of the output MOS transistor 64 and the load 68. The output MOS transistor 64 is turned on or off according to a control signal 63 inputted to its gate terminal.
The current limiter also has a current detection MOS transistor 65. A given ratio exists between the current detection MOS transistor 65 and the output MOS transistor 64. With this current ratio, the current detection MOS transistor 65 monitors the current of the output MOS transistor 64. A control resistor 67 or a control MOS transistor 611 whose drain and gate are short-circuited is connected between the source of the current detection MOS transistor 65 and the output terminal 69.
A protection MOS transistor 66 is connected between the node A and the output terminal 69. The gate of the protection MOS transistor 66 is connected to the source of the current detection MOS transistor 65.
The circuit operation is explained below. The operation when the output MOS transistor 64 is on is as follows. The control signal 63 is set higher than the first power supply voltage by a booster so as to reduce the on-resistance of the output MOS transistor 64. If the voltage of the node A is higher than the voltage of the first power supply 61 and the drain-source voltage of the output MOS transistor 64 is high, the drain current of the output MOS transistor 64 increases. At this time, a current proportional to the drain current flows into the current detection MOS transistor 65. Thus, the voltage of the control resistor 67 or the drain-source voltage of the control MOS transistor 611 is the gate voltage of the protection MOS transistor 66. The current thereby flows through the protection MOS transistor 66 to reduce the voltage at the gate of the output MOS transistor 64 or at the node A. Since the voltage of the node A decreases, the current of the output MOS transistor 64 decreases accordingly. In this way, the circuits of FIGS. 6 and 7 operate as a current limiter. Japanese Unexamined Patent Publication No. 02-226808 discloses this kind of technique.
If the drain-source voltage of the power output MOS transistor increases, the energy in the output MOS transistor increases accordingly, which can ultimately cause the self-destruction of the output MOS transistor. To avoid this, it is necessary to reduce the gate voltage of the output MOS transistor to limit the output current. However, merely reducing the output current results in decrease in the obtained output current, impeding the power output MOS transistor from functioning satisfactory.
In order to obtain a maximum output current without destructing the output MOS transistor, it is preferred to increase the current limit value when the drain-source voltage of the output MOS transistor is low, and decrease the current limit value when the voltage is high. Thus, a current limiter that controls a current depending on the drain-source voltage is required.
A ratio of the current flowing through the output MOS transistor 64 and the current detection MOS transistor 65 is constant. However, to be exact, since the source voltage of the current detection MOS transistor 65 increases as the current of the output MOS transistor 64 increases, the drain-source voltage of the current detection MOS transistor 65 becomes smaller with respect to the drain-source voltage of the output MOS transistor 64. Thus, as the drain-source voltage of the output MOS transistor 64 increases, the current flowing through the current detection MOS transistor 65 decreases accordingly, thereby reducing the current flowing into the protection MOS transistor 66.
This results in increase in the voltage of the input terminal 63, which reduces the current limit value. The current characteristics of the output MOS transistor 64 with respect to the voltage between the first power supply 61 and the output terminal 69 is shown in FIG. 8. As the voltage between the drain and source of the output MOS transistor 64 increases, the energy in the output MOS transistor 64 increases. Thus, a high limiting current causes destruction of the output MOS transistor 64.
Hence, it is preferred to reduce the current limit value as the drain-source voltage of the output MOS transistor 64 increases. FIG. 9 shows a limiting current waveform in the case of using gradual control and the like. This technique, however, requires a large size of circuit and fails to obtain a smooth waveform of the limiting current.